Conventional multi-user communications system use frame-based (or packet-based) transmission to communication between two or more users over a shared channel based on Orthogonal Frequency Division Multiplexing (OFDM)—(OFDM is also sometimes referred to as multicarrier modulation.) A packet is usually formed by a preamble, header, and payload, and transmitted using time-sharing or contention-based media access methods. An example of such a system includes IEEE 802.11 (Wireless LAN), IEEE 802.16 (WiMAX), and ITU G.9960 (G.hn). These systems use OFDM transmission, which is also sometimes referred to as Discrete Multi-Tone (DMT) which divides the transmission frequency band into multiple subcarriers (also referred to as tones or sub-channels), with each sub-carrier individually modulating a bit or a collection of bits.
The header contains important control information for the receiver to decode the payload properly, and also provides information about the packet length for virtual carrier sensing. Hence, it is essential to decode the header reliably. In G.9960, which is incorporated herein by reference in its entirety, and should be familiar to those skilled in the art, the header containing PHYH bits (header information block) is carried over one or two OFDM symbols (D=1 or 2), and within each symbol, multiple header information blocks are repeated over the entire frequency band. (See Editor for G.9960, “ITU-T Recommendation G.9960: Next generation wire-line based home networking transceivers—Foundation,” ITU-T SG15/Q4, January 2009) The default value of D is 1, but expanding it to 2 in some cases is under discussion. See, for example, “G.hn: PHY-Frame Header Extension,” ITU Temporary Document ITU-T SG-15/Q4 09CC-046, August 2009, and ITU Temporary Document ITU-T SG15/Q4 09XC-100 entitled “G.hn: Using Two Symbols for the Header of PHY Frame on Coax,” July, 2009, filed in the priority application and incorporated herein by reference in their entirety.
The possibility of carrying more than PHYH bits in the header (H=1 or 2) is also under discussion as disclosed in the “G.hn: PHY-Frame Header Extension” article and in “G.hn: Extended PHY Frame Header,” ITU-T SG15/Q4 09XC-119, July 2009, filed in the priority application and incorporated herein by reference in their entirety.